By using mouse models, the molecules that affect the development of mammary tumors can be identified and manipulated. Various stages in the pathway to neoplasia can be characterized, including the initial growth dysregulation that serves as a precursor for tumor cells in both mouse and man. Heparan sulfate proteoglycans bind many extracellular molecules, and affect the function of at least some of them, including FGF. The cell surface heparan sulfate proteoglycan syndecan-1 (Sdc1) has been shown to collaborate with the oncogene Wnt-1 to induce hyperplasia and subsequently tumors in mouse mammary glands (Alexander et al, 2000. Nature Genetics 25, p329). Given the widespread dysregulation of Writ signaling pathways implicated in human carcinomas of many different origins, this is an important genetic interaction to characterize at the molecular level. Therefore, we aim to determine how Sdc1 interacts with the Wnt signaling pathway, using transgenic mice and genetically manipulated primary mammary epithelial cells. Mammary glands will be reconstituted in vivo by inoculating transgenic cells into host fat pads, and the activity of the Wnt signaling pathway assessed by measuring epithelial hyperplasia. This novel method conveniently tests the interaction of multiple gene products in vivo. Thus, the growth and dysregulation of Sdc1+/+ and -/- mammary epithelial cells, expressing various Sdc1 and Wnt signaling mutants, will be assessed both in vitro and in vivo. Soluble Sdc1 promotes Wnt signaling in a Drosophila culture model, and may function as a collaborator to augment signaling from the cell surface receptor complex. We will test this hypothesis by examining the epistatic interaction of Sdc1 with the primary Wnt signaling transducer, beta-catenin, and by testing for a direct interaction of Sdc1 with Wnt ligands. Sdc1- /- mice will be infected with mouse mammary tumor virus (MMTV), a mutagen that can be used to identify oncogenic loci, in order to evaluate the relative oncogenicity of specific loci and to broaden the test of tumor susceptibility in this background. In summary, this proposal aims to establish the molecular mechanism underlying the collaboration between Sdc1 and the Wnt signaling pathway that has been demonstrated in mice and may be important to man.